Effect of thiols on macroconidia of Fusarium sulphureum.

Treatment of Fusarium sulphureum macroconidial cells with five thiols alters their morphology. Macroconidial cells incubated in dithiothreitol (DTT), dithioerythritol (DTE), or thiourea differentiate into thick-walled, chlamydospore-like cells (thiol-induced spores). These cells appear similar in size and shape to chlamydospores in the light microscope, but differ markedly in cell wall structure when viewed in the electron microscope (EM). Incubation of macroconidia with both DTT and DTE also leads to the formation of large swollen cells (giant cells) which have a parietal cytoplasm and electron-tranparent cell walls; most of these giant cells lyse within 3 to 7 days of incubation. Thiourea-induced spores are characterized by the deposition of a thick, electron-dense, extracellular layer and an accumulation of mitochondria. DTT and DTE, at the concentrations used, inhibit macroconidial germination while thiourea, mercaptoethanol, and cysteine do not. With the latter three thiols, the newly formed hyphal cells become elongated with either one or both ends swollen. Mercaptoethanol-treated cells contain an abundance of mitochondria. The DTT-induced spore differs from both macroconidia and chlamydospores with respect to cellular lipid and cell wall composition. While the thiols have different effects on the macroconidia, the fact that they all induce cell expansion suggests that they react at some common sites.     

The effect of solubilisation on the character of an ethylene-binding site from Phaseolus vulgaris L. cotyledons

The ethylene-binding site (EBS) from Phaseolus vulgaris cv. Canadian Wonder cotyledons can be solubilised from 96,000 g pelleted material by Triton X-100 or sodium cholate. Extraction of 96,000 g pellets with acetone, butanol or butanol and ether results in a total loss of ethylene-binding activity. Like the membrane-bound form, the solubilised EBS has an apparent K_D(liquid) of 10^-10 M at a concentration of 32 pmol EBS per gram tissue fresh weight. Propylene and acetylene act as competitive inhibitors, carbon dioxide appears to promote ethylene binding and ethane has no significant effect. The solubilised EBS is completely denatured affect. The solubilised EBS is completely denatured after 10 min at 70°C, by 1 mM mercaptoethanol and 0.1 mM dithiothreitol, but not by trypsin or chymotrypsin. However, solubilisation decreases the rate constant of association from 10³ M^-1 s^-1 to 10¹–10² M^-1 s^-1 and hence does not permit experimental determination of the rate constant of dissociation. The pH optimum for ethylene binding is altered from the range pH 7–10 in the membrane-bound form to the pH range 4–7 in the solubilised form. The EBS appears to be a hydrophobic, intergral membrane protein, which requires a hydrophobic environment to retain its activity. Partitioning of the EBS into polymer phases is determined by the detergent used for solubilisation indicating that when solubilised, the EBS forms a complex with detergent molecules.Abbreviations EBS ethylene-binding site - PEG polyethylene glycol     

Bisulfite interacts with binding sites of the auxin-transport inhibitor N-1-naphthylphthalamic acid

The affinity of the auxin-transport inhibitor N-1-naphthylphthalamic acid (NPA) for membrane particles as well as for solubilized binding sites from Cucurbita pepo L. hypocotyls was reduced by low concentrations of bisulfite (half-maximal inhibition at 2·10^-3–3·10^-3 M). Two membrane fractions obtained by sedimentation aided with polyethylene glycol showed differential sensitivity to bisulfite. Other oxidizing or reducing substances tested at 1 mM had no effect, except for N-ethylmaleimide (80% inhibition) and iodine (complete inhibition), both of which reduced the number of binding sites but not their affinity. Addition of bisulfite to either the isoalloxane ring of flavoproteins or to pyridoxal phosphate or quinones is proposed as a possible mechanism of action. Sulfur dioxide, at concentrations measured in polluted air, can lead to bisulfite concentrations in plant tissue sufficient to interfere with NPA-binding sites and hence with auxin transport.Abbreviations DTE dithioerythritol - DTT dithiothreitol - IC₅₀ concentration of half-maximal inhibition - NAA 1-naphthylacetic acid - NEM N-ethylmaleimide - NPA N-1-naphthylphthalamic acid - PEG polyethylene glycol, 6000 molecular weight     

Skin sulfhydryl oxidase. Purification and some properties

A sulfhydryl-oxidizing enzyme has been found in skin of young rats and a method for purifying the enzyme over 600-fold has been developed. Enzymatic activity was assayed either by its ability to oxidize dithiothreitol of by measuring its ability to renature reductively denatured ribonuclease A. Skin sulfhydryl oxidase catalyzed the oxidation of various thiols: dithiothreitol, dithioerythritol, D-penicillamine, and L-cysteine. Glutathione and 2-mercaptoethanol were very poor substrates for the enzyme. The enzyme also reactivated reductively denatured ribonuclease A, with neither the presence of a thiol nor prior reduction of the enzyme being necessary. The molecular weight of the enzyme was estimated to be 66 000 +/- 2000, and the isoelectric point was determined to be at pH 4.65. Alkylating reagents alone had some inhibiting effect on skin sulfhydryl oxidase; when the enzyme was preincubated with thiols which were substrates, inhibition by alkylating reagents was greatly increased. After preincubation with dithiothreitol, treatment of the enzyme with alkylating reagents or N-ethylmaleimide caused significant inhibition; preincubation with a poor substrate, reduced glutathione, did not enhance inhibition by alkylating reagents or N-ethylmaleimide.     

Binding and structural properties of oxytocin receptors in isolated rat epididymal adipocytes

Oxytocin initiates its insulin-like action in adipocytes through oxytocin-specific receptors. We have studied binding and structural properties of these receptors with the radioligand [3H]oxytocin. Steady-state binding was reached after 45 min, at 21 degrees C, and 10 min at 37 degrees C. Scatchard analyses of equilibrium binding data indicated a single class of oxytocin binding sites at 21 degrees C (KD = 3.3 nM, RT = 6 X 10(4) sites/cell) and 2 binding sites at 37 degrees C (KD = 1.5 nM, RT = 6 X 10(4) sites/cell; and KD = 20 nM, RT = 30 X 10(4) sites/cell). Insulin, insulin-like growth factor I, and epidermal growth factor increased oxytocin binding (approximately 20-40%), whereas adenosine, a regulator of oxytocin action, did not affect oxytocin binding. Binding activity of oxytocin was impaired by pretreatment of the hormone or adipocytes with dithiothreitol. Dithiothreitol treatment of adipocytes preferentially inactivated high-affinity binding sites. N-ethyl maleimide inhibited oxytocin binding in adipocytes more than dithiothreitol. In contrast to the inhibitory effects of dithiothreitol and N-ethyl maleimide, proteases (trypsin, chymotrypsin and papain) were not able to inhibit fat cell binding activity. These results suggested that in isolated adipocytes: there are high-affinity and low-affinity receptors, but the low-affinity receptors are absent at 21 degrees C; the binding of oxytocin can be regulated by insulin, and growth factors; and the oxytocin receptors contain disulfide bridges and free thiols that are essential for the maintenance of oxytocin binding.     

Involvement of sulfhydryl groups in the activation mechanism of the ATPase activity of chloroplast coupling factor 1

Activation of the ATPase activity and the exposition of a new adenine nucleotide binding site of chloroplast coupling factor 1 (CF1) by dithioerythritol at 25 degrees C were reversed by oxidants. The ATPase activity elicited by heat (63 degrees C, 4 min) was slightly inhibited by oxidants and was partially additive with the activity induced by dithioerythritol. Titration of the thiols of CF1 and determination of their subunit distribution before and after activation by dithioerythritol show an increase of the free groups from 8 to 10 with the appearance of the 2 new thiols on the gamma subunit. These thiols were available to reagents in nondenatured enzyme and were reoxidized to a disulfide bond by iodosobenzoate or CuCl2. It is concluded that the mechanisms of CF1 activation by dithioerythritol and by heat are different and that the former involves a net reduction of a disulfide bond of the gamma subunit.     

The effect of light on the production of ethylene from 1-aminocyclopropane-1-carboxylic acid by leaves

White light inhibits the conversion of 1-amino-cyclopropane-1-carboxylic acid (ACC) in discs of green leaves of tobacco (Nicotiana tabacum L.) and segments of oat (Avena sativa L.) leaves by from 60 to 90%. Etiolated oat leaves do not show this effect. The general nature of the effect is shown by its presence in both a mono- and a dicotyledon. Since the leaves have been grown and pre-incubated in light, yet can produce from 2 to 9 times as much ethylene in the dark as in the light, it follows that the light inhibition is fully reversible. The inhibition by light is about equal to that exerted in the dark by CoCl₂; it can be partly reversed by dithiothreitol and completely by mercaptoethanol. Thus the light is probably acting, via the photosynthetic system, on the SH group(s) of the enzyme system converting ACC to ethylene.Abbreviation ACC 1-aminocyclopropane-1-carboxylic acid     

Properties of latent and thiol-activated rat hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase and regulation of enzyme activity

The effect of the thiols glutathione (GSH), dithiothreitol (DTT), and dithioerythritol (DTE) on the conversion of an inactive, latent form (El) of rat liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.34) to a catalyticaly active form (Ea) is examined. Latent hepatic microsomal HMG-CoA reductase is activated to a similar degree of activation by DTT and DTE and to a lower extent by GSH. All three thiols affect both Km and Vmax values of the enzyme toward HMG-CoA and NADPH. Studies of the effect of DTT on the affinity binding of HMG-CoA reductase to agarose-hexane-HMG-CoA (AG-HMG-CoA) resin shows that thiols are necessary for the binding of the enzyme to the resin. Removal of DTT from AG-HMG-CoA-bound soluble Ea (active enzyme) does not cause dissociation of the enzyme from the resin at low salt concentrations. Substitution of DTT by NADPH does not promote binding of soluble El (latent enzyme) to AG-HMG-CoA. The enzymatic activity of Ea in the presence of DTT and GSH indicates that these thiols compete for the same binding site on the enzyme. Diethylene glycol disulfide (ESSE) and glutathione disulfide (GSSG) inhibit the activity of Ea. ESSE is more effective for the inhibition of Ea than GSSG, causing a higher degree of maximal inhibition and affecting the enzymatic activity at lower concentrations. A method is described for the rapid conversion of soluble purified Ea to El using gel-filtration chromatography on Bio-Gel P-4 columns. These combined results point to the importance of the thiol/disulfide ratio for the modulation of hepatic HMG-CoA reductase activity.     

The effect of thiols onSaccharomyces fragilis

Treatment of cell suspensions ofSaccharomyces fragilis with 0.01m β-mercaptoethanol or dithiothreitol released a variety of substances of high and low molecular weight. Twenty-two high-molecular-weight glycoproteins were separated by a combination of chromatography on DEAE cellulose and polyacrylamide gel electrophoresis in presence of sodium dodecylsulphate. The carbohydrate components consisted of at least 95% mannose and the protein components had threonine and serine as the major amino acids. Only very small amounts of phosphorus were associated with the high-molecular-weight components. The low-molecular-weight substances were probably released from the internal cell pool and uracil and hypoxanthine were identified as components of this fraction. It is suggested that in addition to breaking disulphide bridges in the cell wall the thiols may also render the plasmalemma permeable to certain low-molecular-weight substances. Such effects are not lethal since the yeast can be trained to grow in presence of 0.01m mercaptoethanol.     

Kinetic mechanism and specificity of bovine milk sulphydryl oxidase

Sulphydryl oxidase is known to catalyse the synthesis de novo of disulphide bonds in a variety of thiol-containing compounds. Reduced glutathione is the best thiol substrate; however, D- and L-cysteine, cysteamine and N-acetyl-L-cysteine, as well as cysteine-containing peptides and proteins, are also effectively oxidized. In contrast, oxidation of the thiol groups of mercaptoethanol, mercaptopyridine, dithiothreitol, dithioerythritol, mercaptoacetate, mercaptopropionate or lipoic acid is not detectably catalysed. In bovine milk, sulphydryl oxidase is closely associated with another glutathione-metabolizing enzyme, gamma-glutamyltransferase. Covalent chromatography of crude preparations on cysteinylsuccinamidopropyl-glass resolves the oxidase from the transferase, thus permitting the kinetic characterization of glutathione oxidation. Initial-rate data imply a Ter Bi substituted-enzyme mechanism, and the observed substrate inhibition by thiols suggest that O2 binds first. Independent, non-kinetic, data, namely the immobilization of sulphydryl oxidase on cysteinyl-matrices, support formation of a mixed-disulphide intermediate between the thiol and enzyme, as predicted by the proposed mechanism. The enzyme-catalysed reaction appears not to be mediated via a superoxide intermediate, since O2 consumption is not affected by the presence of Nitro Blue Tetrazolium. FAD, NAD+, NADP+ and Nitro Blue Tetrazolium are all inactive as electron acceptors for sulphydryl oxidase catalysis.     

p-Chloromercuribenzoate Causes Ca2+-Dependent Exocytotic Catecholamine Secretion from Cultured Bovine Adrenal Medullary Cells

Incubation of cultured bovine adrenal medullary cells with p-chloromercuribenzoate (50-500 microM), a sulfhydryl-reacting agent, caused an increase in the secretion of catecholamines, p-Chloromercuriphenyl sulfonate, a p-chloromercuribenzoate analogue that poorly penetrates the cell membrane, caused a similar increase in catecholamine secretion. In both cases, catecholamine secretion was dependent on extracellular Ca2+. Furthermore, p-chloromercuribenzoate caused both 45Ca2+ influx into the cells and an increase in the intracellular free Ca2+ concentration. The increases in catecholamine secretion and 45Ca2+ influx behaved similarly in relation to p-chloromercuribenzoate concentration. The time courses of the increased secretion, 45Ca2+ influx, and intracellular free Ca2+ concentration by p-chloromercuribenzoate were also quite similar. The stimulation of catecholamine secretion by p-chloromercuribenzoate was reversed by washing the cells with dithiothreitol-containing medium, but not by dithiothreitol-free medium. When the cells were treated with p-chloromercuribenzoate, dopamine-beta-hydroxylase, an enzyme present in the chromaffin granules along with catecholamines, was also released. However, p-chloromercuribenzoate did not cause release of phenylethanolamine-N-methyltransferase, an enzyme present in the cytoplasm. These results indicate that catecholamine secretion due to p-chloromercuribenzoate occurs by Ca2+-dependent exocytosis.     

Sulfhydryls of platelet tubulin: their role in polymerization and colchicine binding

Sulfhydryls and disulfides of platelet tubulin have been quantified, their accessibility and reactivity measured, and their role in polymerization and colchicine binding evaluated. Platelet tubulin isolated by two cycles of temperature-dependent polymerization--depolymerization was found to contain 12 free sulfhydryl groups per tubulin monomer all of which reacted rapidly with p-chloromercuribenzoate. One sulfhydryl was inaccessible to dithiobis(nitrobenzoic acid). Under anaerobic conditions of tubulin extraction, one intrachain disulfide bridge was found per tubulin monomer. Polymerization of tubulin reduced the number of sulfhydryls by one which were able to react with p-chloromercuribenzoate or dithiobis(nicotinic acid) but did not affect the disulfide bridge. Polymerizability of platelet tubulin was very sensitive to blocking of free sulfhydryl groups. Complete inhibition of microtubule assembly was obtained when the number of free sulfhydryls per tubulin was reduced by 3 but could be reversed by the addition of dithiothreitol. Colchicine binding, on the other hand, was only minimally influenced by blocking of sulfhydryls.     

On the Nature of the Interaction Between Serotonin and Serotonin Binding Protein: Effect of Nucleotides, Ions, and Sulfhydryl Reagents

Abstract: Rat brain serotonin binding protein (SBP) was found to have essential -S-S and -SH groups. Both reduction of the disulfide bond by dithiothreitol or mercaptoethanol and modification of -SH group(s) by Ellman reagent or alkylating agents caused loss of binding capacity. In contrast, formation of a mixed disulfide bond with sodium metabisulfite did not affect the binding capacity. Serotonin in the presence of Fe²⁺ and phosphate was found to bind to either an -SH group or to a site in very close proximity. Addition of serotonin protected -SH groups from modification by Ellman reagent and from denaturation of protein upon storage. Lipids that enhance binding of serotonin to SBP also protected -SH groups from modification. Nucleotides were found to be strong inhibitors of the binding of serotonin to SBP. The inhibitory effect of nucleotides was due to their chelating properties and not to their ability to phosphorylate the protein or to bind directly to it. Inhibition by nucleotides and other chelators was reversible. Binding capacity was fully restored after removal of the chelator by molecular sieve chromatography and addition of Fe²⁺. The ionic environment had a marked effect on the binding: intracellular ions such as K⁺ were found to enhance the binding, and extracellular ions such as Na⁺ and Ca²⁺ inhibited the binding. Based on these data and our previous studies, we suggest that SBP is an intracellular protein that acts as a storage protein. Consistent with our data is formation of a complex of SBP-S-Fe-S that in a hydrophobic surrounding could bind up to four molecules of serotonin in coordination bond with Fe²⁺ and thereby reduce the osmotic pressure within a storage vesicle. Extracellular ionic conditions that favor the dissociation of the complex would free the amine to interact with its receptor or the presynaptic reuptake carrier.     

Ethylene binding sites in higher plants

A review of work carried out on ethylene binding in higher plants is presented. The use of radio-labelled displacement assays has identified specific ¹⁴C-ethylene binding in all tissues so far studied. virtually all higher plants studied contain at least two classes of ethylene binding site, one of which fully associates and dissociates in about 2 h and a class of sites that takes up to 20 h to become fully saturated. Although the types of site differ in their rate constants of association they have similar and high affinities for ethylene.A series of Arabidopsis thaliana mutants shown to vary in sensitivity to ethylene have been analysed for ¹⁴C-ethylene binding. One mutant, eti 5, which was shown to be unaffected by ethylene concentrations of up to 10,000 L L^–1 was also shown to exhibit reduced binding. In vivo and in vitro studies on pea have shown that ethylene binding can be detected in this tissue. In vitro studies have shown that both membrane and cytosolic fractions contain measurable amounts of ethylene binding. Interestingly, cytosolic ethylene binding consisted only of the fast associating/dissociating type.Developing cotyledons of Phaseolus vulgaris contain a higher concentration of ethylene binding sites that other tissues and only contain the slow dissociating component. These facets have allowed the purification of ethylene binding protein(s) (EBP) from this tissue. The proteins which bind ethylene can be resolved into two bands of 26 and 28 kDa on semi-denaturing PAGE and the proteins appear to be single entities on a 2-D gels.Data will be presented which indicate a possible role for heterotrimetric G-proteins in the early stages of the ethylene signal transduction pathway.     

Biological thiols elicit prolactin proteolysis by glandular kallikrein and permit regulation by biochemical pathways linked to redox control

Rat glandular kallikrein (GK), a trypsin-like serine protease, cleaves rat prolactin (PRL) in vitro to novel forms detectable in vivo and likely to be of physiological significance. PRL proteolysis by GK is thiol-dependent, with thiols acting upon PRL to refold the molecule into novel conformations that are GK substrates. This study compared several natural and synthetic thiols for their ability to elicit PRL proteolysis by GK. Rat PRL was incubated with rat GK in the presence of various thiols and 0.5% Triton X-100, which enhances thiol-elicited proteolysis. Cleavage was analyzed by gel electrophoresis under reducing and nonreducing conditions. In the presence of Triton X-100, all low molecular weight thiols elicited PRL cleavage by GK. The order of potency was dithiothreitol greater than mercaptoethanol greater than lipoic acid greater than cysteamine = glutathione (GSH) = coenzyme A greater than cysteine. In the absence of Triton, however, dithiothreitol, coenzyme A, and mercaptoethanol were most effective in eliciting GK proteolysis. Triton X-100 enhanced PRL cleavage by 4-19-fold, depending upon the thiol used. Folding isomers of processed PRL observed following cleavage included disulfide-liked homodimers, oxidized monomers, reduced monomers and mixed disulfides; the folding isomers generated varied depending upon the thiol used. GSH potency in eliciting PRL proteolysis increased 10-fold in the presence of biochemical pathways shuttling reducing equivalents to GSH disulfide (GSSG). PRL cleavage by GK could be controlled by substrates, enzymes, and cofactors making up the reducing shuttle when GSSG was used. Thioredoxin (a protein disulfide oxidoreductase) potently elicited PRL proteolysis by GK in the presence of a reducing shuttle and Triton X-100.(ABSTRACT TRUNCATED AT 250 WORDS)     

Auxin binding to subcellular fractions from Cucurbita hypocotyls: In vitro evidence for an auxin transport carrier

An auxin binding sive, with characteristics different from the previously described auxin binding sites I and II in maize coleoptiles, is reported in homogenates of zucchini (Cucurbita pepo L. cv. Black Beauty) hypocotyls. Evidence from differential centrifugation and sucrose and metrizamide density gradients indicates that the site is localized on the plasma membrane. The site has a K_D of 1–2×10^–6 M for indole acetic acid and has a pH optimum of 5.0. Binding specificity measured with several auxins, weak auxins, and anti-auxins generally parallels the activities of the same compounds as inhibitors of auxin transport. 1-N-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid (2,3,5-TIBA), both auxin transport inhibitors in vivo, increase specific auxin binding to this site. 3,4,5-TIBA, which can partially reverse 2,3,5-TIBA's transport inhibition when the two substances are added together in vivo, partially reverses 2,3,5-TIBA's increase in specific auxin binding to the plasma membrane site when added with 2,3,5-TIBA in vitro. Preliminary investigations indicate that a similar plasma membrane site exists in maize (Zea mays L.) coleoptiles. It is suggested that different conformations of this site may function during active auxin transport.Abbreviations IAA indole-3-acetic acid - NPA 1-N-naphthylphthalamie acid - 2,3,5-TIBA 2,3,5-triiodobenzoic acid - 3,4,5-TIBA 3,4,5-triiodobenzoic acid - 1-NAA 1-naphthaleneacetic acid - 2-NAA 2-naphthaleneacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - DTE dithioerythritol - MOPS N-morpholino-3-propansulfonic acid - CCO cytochrome c oxidase - CCR NADH: cytochrome c reductase - glu I glucan synthetase I - ER endoplasmic reticulum     

Rapidly induced ethylene formation after wounding is controlled by the regulation of 1-aminocyclopropane-1-carboxylic acid synthesis

Bean leaves from Phaseolus vulgaris L. var. Pinto 111 react to mechanical wounding with the formation of ethylene. The substrate for wound ethylene is 1-aminocyclopropane-1-carboxylic acid (ACC). It is not set free by decompartmentation but is newly synthesized. ACC synthesis starts 8 to 10 min after wounding at 28°C, and 15 to 20 min after wounding at 20°C. Aminoethoxyvinylglycine (AVG), a potent inhibitor of ethylene formation from methionine via ACC, inhibits wound ethylene synthesis by about 95% when applied directly after wounding (incubations at 20°C). AVG also inhibits the accumulation of ACC in wounded tissue. AVG does not inhibit conversion of ACC to ethylene. Wound ethylene production is also inhibited by cycloheximide, n-propyl gallate, and ethylenediaminetetraacetic acid.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - AVG ammoethoxyvinylglycine - EDTA ethylenediaminetetraacetic acid     

Contrasting effects of various sulfhydryl reagents on the activity of lecithin: Cholesterol acyltransferase

Lecithin:cholesterol acyltransferase, E.C. 2.3.1.43, can be either stimulated or inhibited by reducing reagents containing free sulfhydryl groups. Mercaptoethanol and dithiothreitol stimulate enzyme activity while cysteine and reduced glutathione inhibit activity. The oxidizing disulfide reagent 2-pyridine disulfide has minimal effects on enzyme activity by itself but suppresses the stimulatory effect of mercaptoethanol.     

tert–butylhydroperoxide—Induced toxicity in isolated hepatocytes: Contribution of thiol oxidation and lipid peroxidation

Incubation of isolated rat hepatocytes with tert-butylhydroperoxide resulted in marked cytotoxicity preceded by intracellular glutathione depletion and extensive lipid peroxidation. Addition of antioxidants delayed, but did not prevent, this toxicity. A significant decrease in protein-free sulfhydryl groups also, occurred in the presence of tert-butylhydroperoxide; direct oxidation of protein thiols and mixed disulfide formation with glutathione were responsible for this decrease. The involvement of protein thiol depletion in tert-butylhydroperoxide–induced cytotoxicity is suggested by our observation that administration of dithiothreitol, which caused re-reduction of the oxidized sulfhydryl groups and mixed disulfides, efficiently protected the cells from toxicity. Moreover, depletion of intracellular glutathione by pretreatment of the hepatocytes with diethyl maleate accelerated and enhanced the depletion of protein thiols induced by tert-butylhydroperoxide and potentiated cell toxicity even in the absence of lipid peroxidation.     

Inhibition of elongation growth by two sesquiterpene lactones isolated from Helianthus annuus L.

Two sesquiterpene lactones belonging to the germacranolides were isolated from the leaves and stems of Helianthus annuus L. Their formation in the plant is light-dependent. Both sesquiterpene lactones (SL) strongly inhibit indole-3-acetic acid (IAA)-induced elongation growth of Avena sativa L. coleoptile segments and Helianthus annuus L. hypocotyl segments. Both SL do not, however, inhibit acid-induced growth nor growth triggered by fusicoccin at all. In the presence of dithiothreitol (DTT), the inhibitory effect of SL in the Avena-segment-test can be completely neutralized. This can be attributed to the binding of DTT to both SL. Using thin-layer-chromatography it could be shown that the inhibitors build adducts with SH-rich compounds, e.g., cysteine, glutathione, mercapto-ethanol, and DTT, whose R_f-value significantly differs from those of the primary substances. If the coleoptile segments are first treated with an inhibitor and the inhibitor is subsequently washed out, close to normal elongation growth can be induced by adding an IAA-solution. If the segments are simultaneously treated with inhibitor and IAA, no notable growth can be initiated for an extended amount of time, after the removal of both substances and the anewed addition of IAA. Fusicoccin, however, can immediately neutralize the induced growth inhibition. The same irreversible inhibition is observed when 2,4-dichlorophenoxyacetic acid (2,4-D) is used: If coleoptile segments are treated with an inhibitor plus 2,4-D or an inhibitor plus 3,5-dichlorophenoxyacetic acid (3,5-D), respectively, IAA-induced growth after removal of the substances can only be observed by those coleoptiles which had previously been treated with the non-auxin, 3,5-D plus an inhibitor. Based on these results, a possible mechanism describing how the inhibitor functions is discussed. The binding of an auxin to an auxin receptor sets a SH-group free (possibly due to a change in the conformation of the receptor); a site is given to which the inhibitor can bind irreversibly (via a S-bond). The IAA-receptor-inhibitor-complex is then no longer able to initiate elongation growth. If auxin is not present, no lasting bond between the inhibitor and the receptor can occur, since the essential SH-group remains masked. The inhibitor can be washed out again. Consequently, the SL's have to be able to intervene at the beginning of the IAA-induced reaction sequence, while the following steps remain uninfluenced, i.e. namely, the active excretion of protons into the cell wall compartments, which is directly induced by fusicoccin and causes elongation growth.Abbreviations 2,4-D 2,4-dichlorophenoxy-acetic acid - 3,5-D 3,5-dichlorophenoxy-acetic acid - DTT dithiothreitol - FC Fusicoccin - GA₃ gibberellic acid - IAA indole-3-acetic acid - MES 2-(N-morpholino)-ethane sulfonic acid - SL sesquiterpene lactone(s)